Manufacture of tetrallkyllead compounds



F 4, 1953 H. M. TANNER 2,635,105 MANUFACTURE OF TETRAALKYLLEAD COMPOUNDS Filed May 25, 1951 2 SHEETS-SHEET 1 5-MlNUTE Y|ELD, PER CENT TETRAETHYLLEAD (ALKYLATIQN RATE OF SODIUM-LEAD ALLOY) I5 SOLIDIFICATION TIME, SODIUM-LEAD ALLOY, MINUTES FIGURE l INVENTOR. HOMER M. TANNER YfMM ATTORNEY Apnl 14, 1953 H. M. TANNER MANUFACTURE OF TETRAALKYLLEAD COMPOUNDS 2 SHEETS SHEET 2 Filed May 25, 1951 SOLIDIFICATION TIME, SODIUM-LEAD ALLOY, MINUTES FIGURE 2 INVENTOR.

HOMER M. TANNER BY W ATTORNEY Patented Apr. 14, i953 MANUFACTURE OF TETRAALKYLLEAD COMPOUNDS Homer M. Tanner, Baton Rouge, La., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware Application May 25, 1951, Serial N 0. 228,236

7 Claims. 1

This invention relates to the manufacture of tetraalkyllead compounds, and more particularly to a new and improved process for the manufacture of tetraalkyllead compounds from lead, sodium and alkylating agents.

Tetraalkyllead compounds are commonly made by the alkylation of a sodium-lead alloy with an alkylating agent, such as an alkyl chloride. Thus, compounds such as tetraethyllead, tetramethyllead, tetraisopropyllead and the like, are made by alkylating an alloy of sodium and lead with an alkyl halide, for exampl methyl bromide, ethyl chloride, ethyl bromide, ethyl iodide and propyl or isopropyl bromide. A commercially important example of such processes is the preparation of tetraethyllead by ethylating monosodium-lead alloy with ethyl chloride, the product receiving Wide usage as an anti-knock component.

' In the past, such processes have been characterized by the casting of such alloys in large masses, followed by the comminution of the resultant pig or ingot, and then the alkylation of the comminuted alloy in autoclaves at an elevated temperature and pressure. As an example of such prior methods, Fielding, in U. S. Patent 1,884,640 describes the casting of sodium-lead alloy in cone-shaped molds, resulting in ingots of about 500 pounds weight. A more eificient method is shown by Stecher, in U. S. Patent 2,134,091. In the Stecher apparatus, a slab of solid alloy is formed by solidifying a charge of molten alloy in a rectangular pan provided with suitable cooling means and means for maintaining an inert gaseous atmosphere or blanket in contact with the alloy. Owing to the mass of alloy prepared per charge, of the order of 1600 pounds, solidification of the liquid charge requires about 20 to 25 minutes. Upon completion of the solidification, the pan is vibrated along a straight path at a slight inclination to the horizontal. The vibration results in the fracture of the solid slab into smaller chunks or masses which are discharged at one end of the apparatus and thence pass to suitable crushers for further comminution. The tetraalkyllead product is then formed by alkylating the sub-divided alloy in an autoclave.

.Whlle such prior methods of making tetraalkyllead compounds have been reasonably satisfactory, they suffer from certain disadvantages which have heretofore been only partially realized. In particular, the alloy so-formed reacts only sluggishly with the alkylating agents, so thatthe time necessaryto obtain a practical limited, since with the slow tetraalkyllead forma# tion rate exhibited by sodium-lead alloys as made by present means, the sodium-lead alloy and alkylating agent must be kept in contact with each other under reaction conditions for an inordinately long period of time. Thus, the number of cycles or charges which can be processed by a batch-type operation, and hence the overall capacity of a commercial installation, is limited by the above-mentioned sluggishness of reaction. Considering a continuous-type reactor, this sluggishness of reaction of conventional-type sodium-lead alloys with an alkylating agent, such as ethyl chloride, means that the rate of feeding of sodium-lead alloy and alkylating agent to the reactor will be limited for a reactor of given size, to the extent that the feed rate must be slow enough so that the residence time necessary for completion of the alkylation reaction is achieved in the reactor.

An object of the present invention is to provide a process whereby tetraalkyllead compounds can be made more emciently and at a faster rate than has heretofor been possible. A specific object is to provide a more efficient, high-capacity process for the manufacture of tetraethyllead. An additional object is to provide a means of increasing the capacity of commercial equipment, whether batch-type or continuous-type, used in the manufacture of tetraalkyllead compounds. A further object is to provide an integrated process for making tetraalkyllead compounds including the preparation of a highly reactive alloy of sodium and lead and the alkylation thereof at a rapid and eflicient rate. A still further object of the invention is to provide a new sodium-lead alloy which is highly reactive under widely varying conditions.

I have now provided a new and improved process whereby the above objects are attained and many of the deficiencies of prior practice are overcome. In its broadest terms, the process comprises the formation of a liquid or molten metal containing sodium and lead, cooling this melt to the solidification temperature, and then removing heat at a rate such that complete solidification is attained in a restricted and finite period, and then alkylating the solid alloy to form the desired tetraalkyllead compound.

Specifically, in order that the advantages and benefits of my invention may be realized, it is necessary that the solidification time of the sodium-lead alloy be confined to a limited period, as hereafter explained. By solidification time, I mean the period of time between the start of formation of a solid phase and the time at which the entire mass of liquid alloy has been transformed into the solid state. In the case of a homogeneous substance, this solidification time is characterized by the length of a horizontal plateau on a conventional cooling curve obtained by plotting temperature of the substance versus time, starting with the substance in the liquid state and allowing it to cool below the temperature of solidification. In the case of a mixture of two substances which form a heterogeneous solid, the solidification time is the time elapsed between two points of discontinuity; the first point of discontinuity occuring when the first particle of solid appears and the second point of discontinuity occurring when the last trace of liquid is converted into solid.

It has been discovered that the reactivity of sodium-lead alloy can be materially increased by solidifying said alloy in a time of not more than about two minutes during which the alloy is maintained in a substantially quiescent or nonagitated state. Alloys which have been solidified in a time greater than the aforementioned time will exhibit only a slight and hardly perceptible advantage in reaction rate over alloys prepared by the conventional method of slowly cooling massive alloy batches. On the other hand, when the solidification, as defined above, is about two minutes or less, then the reactivity of the alloy is sharply and markedly enhanced, so that tetraethyllead is produced at a much more rapid rate than heretofore.

While the benefits of the process are obtained in all embodiments wherein the alloy solidification period is about two minutes or less, it has been further found that the degree of improvement varies within this range of operation. Thus, to achieve the maximum degree of improvement, it has been found that a solidification time of less than about'30 seconds is desirable. Accordingly, in the preferred embodiments of the process, heat is abstracted from the liquid alloy at a rate such that the solidification time is not over 30 seconds.

The reason for the improvements effected by the process, in particular the increase in the alkylation rate obtained, is not fully understood. It has been noted, however, that the effective alloys made by the present method are all characterized by being non-porous solids, even upon microscopic examination. By non-porous, is meant that there are no visible apertures or crevices in the alloy surfaces. Such non-porous solids are obtained by refraining from any comminuting action during the alloy solidification period; that is, the conversion from liquid to solid phases is effected while the alloy is maintained in a quiescent or non-agitated condition.

Such oomminution as is required, primarily to facilitate the mechanical handling of the alloy, is deferred until completion of the solidification period. In most embodiments of the process, a comminuting operation will be included, but excessive particle size reduction is to be avoided. It has been found that the benefits of the process are substantially negated if more than per cent of the particles are reduced in size sufficiently to pass a 100 mesh screen. By 100 mesh screen, I mean the screen designated as the 100 mesh screen in the U. S. Sieve Series (see Dallavalle. Micromeritics." Pitman Publishing Co.,

New York, N. Y., 1943, p. 83). This occurs despitethe fact that the usual precautions are taken for protecting the alloy from exposure. In particular, even when the alloy is continuously blanketed or protected by a dry atmosphere of nitrogen gas, such portions of the comminuted alloy as will pass a 100 mesh screen are deprived of the desired attributes of high reactivity or sus ceptibility of rapid alkylation. Thus our alloy size should have per cent of the particles larger than mesh and preferably over 50 per cent larger than 50 mesh.

In the accompanying figures, Figure 1 is a graph showing the variation in the alkylation rate of a sodium-lead alloy as affected by the solidification period. Figure 2 more precisely shows the effect of the duration of the solidification period, being a plot of the negative value of the first derivative of the alkylation rate as it varies with the solidification period.

The following examples, to allow adequate comparison, are illustrations of the application of the process to the preparation of tetraethyllead wherein the sodium and lead are alloyed as monosodium-lead alloy and then ethylated with ethyl chloride.

The following example illustrates the preparation of tetraethyllead in a manner comparable to prior practice; in particular, in that an extended solidification period was employed in making the alloy. In this example and in others hereafter given, all quantities or compositions are in parts or percentages by weight based on the alloy charged except when otherwise stated.

Example I A liquid sodium-lead alloy was prepared by melting together 10 parts of metallic sodium and 90 parts of lead metal, the melting and subsequent treatment being done with only dry, pure nitrogen in contact with the metal. The soformed liquid monosodium-lead alloy was cooled from the initial temperature of 400 C. to the solidification temperature of the monosodium alloy. Further heat removal was then controlled at such a rate that the solidification period was 23 minutes. In this cooling cycle, the temperature of the alloy was carefully observed at measured time intervals by means of a thermocouple actually inserted in the alloy mass. The thermocouple in this instance was located within a distance of three-eighths of an inch of the heat removal boundary of the alloy.

Upon completion of the solidification of the alloy, the solid mass was fractured into smaller particles, none of which, however, were smaller than about one-sixteenth of an inch in average diameter. A portion of the alloy was then alkylated with ethyl chloride, in the ratio of 50 parts of alloy to 100 parts of ethyl chloride. The ethylation was carried out at a temperature of 85 C. and was terminated after precisely five minutes at this temperature by quenching the ethylation reactor or autoclave in a bath maintained at'a temperature of -78 C. It has been well established that such a brief and carefully controlled alkylation period in preparation of tetraalkyllead compounds provides a realistic evaluation of the rapidity of reaction. The reaction products mixture was then extracted with benzene and the tetraethyllead so isolated was determined by reaction with an excess of iodine and back titration of excess iodine reagent. 'This analysis showed a yield of 51 per cent tetraethyllead, based upon a tains the alloy charge,- tion:

In this operation, and also in the subsequent examples, acetone was added to the ethyl chloride in the concentration of 1.5 per cent. It has been found, from a large number of carefully observed ethylations, that this catalyst is of benefit in assuring reproducible results, that is, by mitigating the effect of the minute variations in technique inevitably occurring in a series of ethylations.

The above example illustrates the rate of formation of tetraethyllead obtainable by the present process-and is comparable to prior methods mentioned heretofore. The following examples show the benefits derived when the alloy solidification time is controlled as by the improved process.

Example II Monosodium-lead alloy was prepared in the same manner as described in the preceding example, except that heat was abstracted sufficient- 1y rapidly to provide a solidification period of two minutes and eighteen seconds. The solid alloy so formed was fractured and ethylated for five minutes as in Example I, a yield of 59 per cent being obtained.

Example III Using the same procedure as in the preceding example, monosodium-lead alloy was prepared and the solidification period was reduced to 25 seconds. Upon ethylation with ethyl chloride for five minutes, as in the preceding examples, a yield of 68 per cent was realized.

Example IV Monosodium-lead alloy was again prepared in a similar manner to the previous examples, except in the present instance, the solidification period was about one second. Upon ethylating the so-solidified alloy for five minutes, using the procedure described above, a tetraethyllead yield of 82 per cent was obtained.

The foregoing examples show the substantial increase in the rate of formation of tetraethyllead, when the only significant factor changed is the rate of solidification of the sodium-lead alloy. Thus, it will be seen that decreasing the solidification time to about two minutes raised the production of tetraethyllead approximately 13 per cent over the production obtained when the alloy had been solidified in a 23 minute period. Further lowering to a 25 second solidification time provided an increase of 33 per cent. Lastly, a solidification period of about one second raised the production of tetraethyllead 61 per cent.

In all the foregoing examples the particle size of the alloy was such that over 50 per cent was retained by a 50-mesh screen and over 90 per cent was retained by a 100-mesh screen and further the alloy was non-porous. As heretofore mentioned, it has been found that the benefits of the process are not realized when the alloys used are either porous or too finely sub-divided. The following example shows the deleterious effect of extreme sub-division.

examples, a sample of monosodium-lead alloy was prepared where solidification time was substantially one second. This alloy sample was thengroundusing a conventional hammer-millaccording to the following equaadapted so as to'permit the use ofan atmosphere of inert, dry nitrogen gas. After being ground, the alloy had a particle size distribution such that over per cent passed a -mesh screen. Upon ethylation in the manner described in the preceding examples, a five-minute tetraethyllead yield of about 45 per cent was obtained.

The benefits of the process, and the importance of the alloy solidification period, are further graphically demonstrated by Figures 1 and 2. Figure 1 shows graphically the effect of variation of solidification time of a monosodium-lead alloy on the ethylation rate with ethyl chloride, expressed as the per cent yield obtained in a five minute reaction period. The chart is based upon a series of operations wherein the ethylation was carried out as in the preceding examples. The solidification period was varied as desired by controlling the rate of heat abstraction, and in accomplishing this variation, several difierent means of cooling were employed, thus showing that the rate of solidification is the essential factor and the method of cooling is relatively unimportant. Referring to the figure, it is seen that the ethylation rate increases only slightly as the solidification period is reduced to about two minutes. At solidification periods below about two minutes, however, the rate increases markedly. Thus, at a solidification period of the order of one second, a yield of over 80 per cent tetraethyllead is obtained.

The importance of alloy solidification time is further illustrated by Figure 2. Referring to the figure, ordinate values of the-negative first derivative of the curve of Figure 1 are plotted against the solidification period. The ordinate values are thus expressed in terms of per cent increase in yield per minute decrease in cooling time. It will be seen that in the portion of the curve between the points A and B, the incremental increase in reactivity with incremental decrease in solidification time of the alloy, is at a low and constant level of approximately 0.4 per cent per minute. The point A corresponds approximately to the solidification time of alloy as prepared by the present day conventional method, while point B corresponds approximately to the upper limit of the solidification period as used in my process. At solidification times shorter than that represented by B, it will be noted that the incremental increase in reaction rate with incremental decrease in solidification time of the alloy is reached when the solidification time is restricted to about 30 seconds or below, shown by the segment from C to D. In other words, as precisely as is determinable for solidification periods of less than about 30 seconds, the rate of increase in reactivity is a constant factor. In the preferred embodiments of the process, therefore, a solidification period of less than about 30 seconds is employed. It will be understood, of course, that in virtually every practical instance, a finite time is required for the complete solidification of the alloy; that is, a solidification period of zero time approaches an impossibility. It will also be apparent, however, that extremely limited solidification periods of the order of a fractionof a second are attainable by the appropriate adjustment of the quantity of alloy in process, by the dimensions of the mass with respect to the average distance from the heat removal plane or surface, and bythe effective temperature 'gradientemployed for the abstraction of the heat of fusion. x lngorderi to. more clearly illustrate 'aim'eans of= amass operating my process, the renewing 11s a exampl of the Iormation oi! tetraethyllead rrom my new monosodium-lead alloy and ethyl chloride.

'E'zample'VI In a suitable'metallic container having present an atmosphere of inert gas, was placed 90 parts of lead and 10 parts of sodium. The container was sealed and then heated by conventional means until the temperature of the contents was in the vicinityof 400 C., as measured by a thermocouple immersed in'the alloy, at which temperature the mixture was in the molten state. The container was inverted several times to insure thorough mixing of the metallic elements. The container was then plunged suddenly into a bathcomposed of brine and ice held at a temperature of approximately l C. The container was removed from the brine, dried externally, and subjected to mechanical shock to loosen the solidified alloy from the walls of the container. The container was opened in an atmosphere of dry, oxygen-free nitrogen gas, and the solidified alloy, in each instance 25 parts, was then placed in a reactor capable of withstanding moderate pressure, and the reactor charged with ethyl chloride, in the ratio of 2 parts of ethyl chloride to 1 part of alloy. The reaction was carried out at 85 C by rotating the reactor in a constant temperature bath for the desired length of time, following which the reaction was stopped by quick immersion of the reactor in a liquid bath held at approximately 70 C., at which temperature the reaction is known not to proceed.

My process may be carried out with any alloy of sodium and lead, including inter-metallic compounds such as NaBb, NaQPbi, NarPb, etc., and also with mixtures not corresponding to intermetallic compounds, such as the mixture usually designated as NazPb, as well as other mixtures of sodium and lead which give alloys whose composition is not readily designated by a chemical formula. The use of my process is most advantageous, however, when the alloy employed is that composed of weight per cent sodium and 90 weight per cent lead, commonly designated as NaPb. The temperature of the liquid alloy prior to the solidification period is unimportant, successful results having been obtained from alloys whose temperature in the liquid state varies from more than 550 C. to less than 468 C. It is essential to my process, however, as heretofore indicated, that the alloy produced be non-porous, and such non-porosity is assured by maintaining a quiescent state during the solidification period. Alloys whose particle size is smaller than the mesh sizes given herein perform in an inferior manner when employed in my process, as was '11- lustrated in Example V. The reason for this great reduction in reactivity 'of this finely divided alloy is not fully ascertained. It is believed that the small particles tend to react extremely readily with impurities, such as oxygen or moisture, in the inert gas used to protect the alloy, so that reaction of the alloy with an alkylating agent is greatly inhibited. Therefore, it is import-ant to my processthat theall'o'y produced be'ofrelatively large particle size: and non-porous, so that the alloy surfaces exposed to reactive contaminant in the protective inert gas is kept to a mini-:

mum.

In my process, the alloy can be reacted with any of a great number of alkylating agents, such as ethyl chloride, ethyl bromide, ethyl iodide, di-

ethyl sulfate, n-propy1 chloride, methyl chloride. etc, with or without the presence of catalysts or reaction accelerators. In general the alkylating agents are esters of inorganic acids and must include the hydrocarbon radicals in question as well as a negative radical which reacts with sodium. My preferred alkylation temperature is in the range of from 40 to 100 C.

Means of accomplishing the rapid solidification of sodium-lead alloy,other than these means given herein, will be apparent to those skilled in the art. Among these means maybe mentioned pass ing of droplets of liquid alloy through an extremely cold inert gas, depositing a thin film of liquid alloy. on a rapidly moving cold surface, dropping intoa liquid, extruding a small-diameter wire of alloy through a cold die and other means in which rapid cooling and non--agit'ation of the alloy is obtained. although cooling of a moving acquiescent stream of alloy is contemplated.

The mode of operation of my invention, as well as the preferred means of carrying it out, have been described in detail above. It is to be understood that the examples given were merely for purposes of illustration, and that my invention is not to be limited thereto, nor to be limited in any way except by the claims which are hereto appended.

'I claim:

1. The process of making a tetraalkyllead compound comprising alkylating a non-porous alloy of sodium and lead, said alloy being characterized by having been solidified in a period of not more than about two minutes, and by having a particle size distribution such that over 50 per cent of the alloy is retained on a 50-mesh screen.

2. The process of making a tetraalkyllead compounds comprising alkylating a non-porous alloy of sodium and lead, said alloy being characterized by having been solidified in a period of not more than about 25 seconds, and by having a particle size distribution such that over 50 per cent of the alloyis retained on a 50-mesh screen.

3. The process of making tetraethyllcad comprising ethylating a non-porous sodium-lead al- 10y, said alloy being characterized by having been solidifiedin a period of not more than about 25 seconds, and by having a particle size distribution such that over per cent of the alloy is retained on a mesh screen.

'4. In a process for the manufacture of tetra alkyllead compounds by alkylating a monosodiumlead alloy, the improvement comprising preparing a non-porous alloy of sodium and lead by mixing liquid sodium and liquid lead in the proper proportions, cooling the liquid mixture to the solidification temperature, then removing heat at such a' rate that the alioy is completely solidified in a period of not more than about two minutes, and comminuting to a particle size distribution whereby over 50 per cent of the alloy retained on a 50 mesh screen.

5. Ina process for the manufacture of tetraethyllead by ethylating a sodium-lead alloy, the improvement comprising preparing a non-porous alloy of sodium and lead, consisting essentially of 10 per cent sodium by weight and 90 per cent lead by weight by mixing liquid sodium and liquid lead in the aforementioned proportions, cooling the mixture to the solidification temperature, then removing heat at such a rate that the alloy is completely solidified in a period of not more than about two minutes during'which the alloyis maintained in acquiescent state, and comminuting to KGDllfiGlfi size distribution whereby over 50 per cent is retained on a 50-mesh screen and over 90 per cent is retained on a IGO-mesh screen.

6. A method of making a new highly reactive non-porous sodium-lead alloy comprising solidifying a molten mixture or" sodium and lead durin a time interval of less than about two minutes during which the mixture is maintained in a .sub stantially quiescent state and thereafter comminuting the so-solidified alloy to a particle size distribution whereby over 50 per cent is retained on a 50-mesh screen and over 90 per cent is retained on a IOU-mesh screen.

7. A new highly reactive, non-porous sodiumlead alloy produced by solidifying a moiten mixture of sodium and lead in a time interval of less References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,974,167 Voorhees Sept. 18, 1934 2,047,391 Stecher et a1 Jul 14, 1936 2,310,866 Nourse Feb. 9, 1943 2,572,887 Stanton Oct. 30, 1951 

1. THE PROCESS OF MAKING A TETRAALKYLEAD COMPOUND COMPRISING ALKYLATING A NON-POROUS ALLOY OF SODIUM AND LEAD, SAID ALLOY BEING CHARACTERIZED BY HAVING BEEN SOLIDIFIED IN A PERIOD OF NOT MORE THAN ABOUT TWO MINUTES, AND BY HAVING A PARTICLE SIZE DISTRIBUTION SUCH THAT OVER 50 PERCENT OF THE ALLOY IS RETAINED ON A 50-MESH SCREEN. 